A goal of early modern artists was mimesis. Artists sought to render in the tradition of Apelles works that could appear so real that they fooled the viewer. Keen observation of the colors of things, not as they "were" but as they "appeared" was essential to this goal, and therefore painters began to use nuance in depicting tonal variation in shaded and lit parts of objects, and to imitate not only color but the effects of luster. To achieve this, painters used many more variations of colorants than are listed in the manuals and treatises of their time. They readily adopted new colorants, appropriating materials from other arts and crafts such as glasses and even powdered metals. New sources of materials such as cobalt and antimony led to production of vast amounts of colorants that were formerly rare. Smalt and Naples yellow and its variants became widely available and used. Full exploitation of the potential of the oil medium to give impasted paint or translucent glazes was aided by production of solvents and diluents, which became more available due in part to improvements in distillation of natural resins and increased mining for petroleum.